1. Field of Invention
The present invention relates to processing low-rank coals in fluidized beds to upgrade them to stabilized clean-burning fuel with high heating values.
2. Background
The upgrading processing of coal can take a number of forms such as drying, pyrolysis and mild gasification.
Coal is dried for a variety of reasons, such as to save on transportation costs, to increase the heating value, to increase the net dollar value, to prevent handling problems caused by freezing weather, to improve coal quality particularly when used for coking, briquetting, and producing chemicals, to improve operating efficiency and reduce maintenance of boilers, and to increase coke oven capacity. However drying of coal causes increased dust formation as the dry coal is more friable. Further readsorption of moisture of dried coals is considered a potential problem.
Dry coal is generally preferred in many coal operations. In World War II the Germans determined that dry coal improved pyrolysis in Lurgi-Spulgas ovens, while the French found that the capacity of coking ovens was increased by using said coal. Thus increased tonnages of dry coal were being sold in the United States up to the early 1970s when stringent emission standards elevated its cost to an uneconomic level.
Another trend in the coal mining industry was its increased mechanization resulting in an increased percentage of coal fines. Because coal fines have a greater relative surface area, they are very susceptible to water adsorption. In order to market such fines, drying was necessary.
Difficulties in coal drying abound. Besides the stringent emissions standards adding an economic burden, numerous explosions and fires have occurred when low cost air is employed as the drying medium. Coal dust fines are more susceptible to dust explosions than are larger particles, and often dry coal is treated with heavy oil before shipping to prevent dust formation and the readsorption of moisture. Adding heavy oil to dry coal is a common method to prevent moisture readsorption and autogenous heating, but in so doing increases operating costs.
Other fluids sometimes employed to treat dried coal to prevent moisture readsorption and autogenous heating include vinyl acetate, vinyl acetate/acrylic polymers, styrene-butadiene, acrylic latex or resins, natural gums or resins, tall oil, neoprene, rubber and the like; however, it is common to keep the halogen content low or preferably none since halogens are detrimental to subsequent boiler operation.
Many proposed processes for upgrading coal involve fine grinding and separations in liquid media. The resulting cleaned coal is difficult to handle using conventional techniques because of fine particles and high moisture contents. Additional drying is sometimes employed; however, moisture readsorption, dust formation with its fire and explosion hazards, and spontaneous heating often result in unstable products.
The general problem of coal drying represents removing three types of moisture: free, physically bound, and chemically bound. Free moisture is found in the very large pores and interstitial spaces of coal and often is removed by mechanical means as it exhibits the normal vapor pressure expected of water at that temperature.
Physically bound moisture is more difficult to remove as it is held tightly in small coal capillaries and pores. Because of this, its vapor pressure and specific heat are reduced over that expected of free moisture.
Chemically bound moisture is characterized by a bonding between surfaces and water. Monolayer and multilayer bonding are commonly identified.
Sometimes a fourth type of moisture is identified which comes from the decomposition of organic compounds. It is really not moisture held in coal but is produced during coal decomposition.
Coal drying is characterized by typical drying curves that exhibit distinct rate regions. Firstly, a transient region occurs as equilibrium conditions are sought while the material heats. This is followed by a largely constant rate portion of drying where the material temperature is relatively constant during the unbound moisture removal, and the drying rate is generally determined from only the particle size and moisture content, be it coal or some other material.
The final region is a period of decreasing rate as the material temperature increases and the physically and chemically bound moisture is removed. For this drying regime the particle size, temperature, and residence time are important parameters. Often the drying rate becomes diffusion controlled, and since diffusivity increases with temperature, higher temperatures are employed to continue drying the materials.
During the constant rate period, the heat and mass transfer rates are directly proportional to the driving forces of temperature gradient and humidity gradient respectively; the appropriate proportionality constants, however, are usually experimentally determined. Maintaining large values of said gradients become important when efficient drying equipment is designed; however, if drying residence time is increased easily, such gradients become less important.
For many coals with higher moisture content, the most important variable is often the degree of fines produced for higher velocity drying gases pick up more such fines.
A variety of drying techniques to upgrade low rank coals include hot water and steam drying under pressure and hot-gas drying using a rotary kiln, Roto-Louvre dryer or a Perry turbulent entrainment dryer. Many coals when dried directly in hot gases readsorb moisture and return to nearly the original equilibrium moisture level. In contrast both steam and hot-water drying reduce moisture readsorption.
Another drying factor for ultra-fine coal besides fines carryover is explosions. Since indirect heating is inefficient as it requires large heat transfer surfaces with a separate heating medium that escalates capital cost and leads to high maintenance requirements and low throughput, ideally an inert atmosphere is needed with a low gas velocity.
After World War Il fluidized bed dryers were adapted to coal drying; however, critical control of both coal and gas flow was required in order to avoid fires and explosions. McNally Flowdryer, Dorr-Oliver Fluo-Solids Dryer, Link-Belt Fluid Flow Dryer, and Heyl and Patterson fluidized bed dryers are all well known.
Typically fluidized bed dryers have a coal-fired zone, using stokers or pulverized coal pneumatically injected, where fluidizing air is heated and its oxygen content reduced. Another zone acts as the dryer where the pressure drop across the gas distributor is large relative to the pressure drop across the bed in order to assure good dryer gas distribution. In some installations, gas from the coal is recycled to further reduce the oxygen concentration. Coal distribution is controlled by a feeder-spreader device, such as a roll feeder, multiple screw feeders, or grate.
These fluidized bed dryers are potentially hazardous when air or mixtures of air and recycled gas are employed. The oxygen concentration is critical to avoid explosive conditions, and special safety equipment, such as sprinkler systems, blowout doors, and automatic fail-safe shutdown devices, is common. Additionally the moisture content of the dry coal is often held to relatively high value of 5-10%, or 0.5-1.0% surface water, to make the drying operation less hazardous and to avoid excessive formation of dust. After removal of the surface water, the rising bed temperature becomes the control parameter to keep it safely below auto-ignition conditions.
Equipment to control particulate emissions from fluidized beds include combinations of cyclones, electrostatic precipitators, bag filters, and wet scrubbers. Cyclones are ineffective with particle sizes below five microns, so their operation is usually restricted to extraction of large particle dust loading prior to removal of fine dust particles by subsequent equipment. However cyclones employed at the gas stream dew point or with water-spraying, are nearly as effective as wet scrubbers. Electrostatic precipitators must be kept free of condensation, and in addition, are subject to malfunctions and frequent maintenance.
Flash dryers use entrained fluidized beds to dry particles under residence times of one second or less. This short residence time gives a high capacity with a low inventory of coal and makes them less hazardous than conventional fluidized bed dryers. Yet particle fines entrainment due to the required high gas velocity is a problem and requires additional separation equipment.
In many instances when further operations are performed on the dried coal, these safety problems are transferred from the dryer region to the other process, such as pyrolysis, and product storage.
Pyrolysis of coal takes many forms often concentrating on the various products of mild gas, hydrocarbon liquids and solid char. Before 1940 many world-wide coal processing plants operating with low temperature pyrolysis produced one of more of these products for the commercial market.
With the advent of an international distribution system for petroleum after World War II, low temperature devolatilization of coal rapidly declined, and many such plants were shutdown. When the petroleum shortages appeared after 1970, increased interest in such processes reappeared; however, with the utilization of modern fluidized bed technology, which featured high sweep gas rates, small particle sizes and allowed rapid heating of high-volatile coals, improved yields occurred.
In the United States the development of synfuel processes occurred after 1960. The COED process used a series of fluidized beds to stepwise carbonize caking coals at higher and higher temperatures. The Clean Coke method utilized a fluidized bed devolatilizer while an entrained bed reactor for flash devolatilization was employed by Occidental.
After 1960 European mild gasification processes did produce briquettes from various fines; however, coal tars from this process produced by flash devolatilization have been of poor quality consisting of heavy, highly aromatic components with high melting points. Further, the high dust content has been an additional problem.
In the United States most recent development concentrated on high temperature, high pressure processes designed to produce maximum yields of liquid and gaseous products; however, economic concerns have not been favorable for commercial exploitation. Currently mild gasification plants are not competitive with petroleum in the United States.
Whereas previously much pyrolysis was designed to obtain maximum yields of liquid and gaseous products, modern operations now concentrate upon well-controlled partial pyrolysis designed to produce selected outputs that are recycled within the process to make the final processed coal product.
Prior art United States patents covering the above mentioned fluidized bed processing concepts of coal drying and coal pyrolysis include:
______________________________________ U.S. Pat. No. Inventor Year ______________________________________ 4,943,367 Nixon et al 1990 4,828,576 Bixel-1 et al 1989 4,783,200 Bixel-2 et al 1988 4,775,390 Bixel-3 1988 4,668,244 Nakamura et al 1987 4,533,438 Michel et al 1985 4,501,551 Riess et al 1985 4,498,905 Skinner 1985 4,495,710 Ottoson 1985 4,421,520 Matthews 1983 4,402,706 Wunderlich 1983 4,401,436 Bonnecaze 1983 4,396,394 Li et al 1983 4,249,909 Comolli 1981 ______________________________________
Referring to the above list. Nixon et al disclose an inert gas fluidized bed flash pyrolysis of coal utilizing high heating rates to produce a desirable tar fraction that is condensed and then coked.
Bixel-1 et al disclose a dried coal process where a treating agent to prevent spontaneous ignition is selected from the group consisting of foots oils, petroleum filtrate, and hydrocracker recycle oil. Bixel-2 et al disclose a dried coal process where a heavy cycle or light cycle oil or slurry oil is employed as a treating agent to prevent spontaneous ignition. Bixel-3 discloses a dried coal process where a treating agent to prevent spontaneous ignition is light cycle oil, heavy cycle oil, clarified slurry oil, a petroleum or coal derived distillate, a solution of durene in gasoline and mixtures of two or more of the preceding.
Nakamura et al disclose a method employing screw reactors with improved sealing between stages for upgrading low rank coal; he uses a carbonizing step after drying and subsequent below 100.degree. C. tar treatment by recycled material. Michel et al disclose a two stage fluidized bed coal drying and pyrolysis process with several heat recovery aspects. Riess et al disclose a fluidized bed method of coal drying to obtain a coal product resisting spontaneous ignition by using fine particle separation and deactivating fluid.
Skinner discloses a method for controlling the dusting tendencies of dried coal by treating with a heavy deactivating oil and a light dedusting oil. Ottoson discloses a process for fluidized bed coal drying where rapid heating to mobilize tar is followed by cooling using a recycle stream. Matthews discloses drying coal and treating it for spontaneous ignition with a deactivating dispersion fluid of milled latex paint type solids emulsified with water.
Wunderlich discloses drying coal and then processing with a controlled oxidation to lower its spontaneous ignition. Bonnecaze discloses drying coal and then cooling it with a controlled stream of water. Li et al disclose drying coal with a reduced tendency to spontaneously ignite by cooling it to below 100.degree. F. Comolli discloses a hot gas, moving bed wicking-up or volitation and recondensation process where coal hydrocarbons prevent moisture readsorption.
In modern times a concern for the discharge of heavy metals and alkalis from coal processing has occurred. For more information see, Jacobsen et al. "The Role of Coal Preparation in the Precombustion Control of Hazardous Air Pollutants", Proceeding of American Mining Congress: Coal Preparation, page 82-99, Cincinnati, Ohio, May 1992.